1. Field of the Invention
It is well known that fruit juices (including tomato juice) and their fermentation products (e.g. grape wine and apple cider) contain substantial quantities of organic acids at least part in the free form. For example, the juice of citrus fruits, pineapples and tomatoes contain citric acid as the principal organic acid; in grape juice and grape wine the principal organic acid is tartaric acid (present as the acid potassium salt); and in apple juice and apple cider the principal organic acid is malic acid. In addition, fruit juice typically has a small amount of succinic acid. It is often desirable to recover free, preferably food-grade, organic acids from said juices, for example, when there is an excess of fruit for food-use, when the end-use can tolerate some removal of organic acids or when the organoleptic, storage and/or processing characteristics of the juice will be improved by partial removal of such acids. In the latter two cases, of course, it is imperative that the food and organoleptic values of the juice should not be impaired for the intended end-use.
It is also well-known that organic acids are produced together with other organic matter by fermentation of suitable substrates with appropriate microorganisms. Typically the organic acids have not been recovered directly as free acid from such fermentations but rather in the form of their calcium, sodium, ammonium or potassium salts. It is often desirable as well to recover free organic acids directly from such fermentations.
Illustrative organic acids include:
Citric acid (i.e., 2-hydroxy-1,2,3-propane tricarboxylic acid, C.sub.6 H.sub.8 O.sub.7, molecular weight about 192.13), which occurs abundantly in citrus fruits, e.g. lemons (4 to 8%), grapefruit (1.2 to 2.1%), tangerines (0.9 to 1.2%), oranges (0.6 to 1.0%) and limes (about 7%). It is also made by certain strains of Aspergillus niger grown on the surface of sucrose-and-salt solutions; by a submerged fermentation process using the same microorganism; by submerged fermentation of either glucose or molasses with an equivalent amount of sugar by species of yeasts (for example, Candida guilliermondii); by fermentation of purified, liquid, normal paraffins by other species of yeast (for example, Candida lipolytica). Citric acid is generally recovered from a fermented aqueous solution by first separating the microorganisms and then precipitating the citrate ion as the insoluble calcium salt to separate fermentation by-products and other impurities from the citrate ion. Acidification with sulfuric acid converts calcium citrate to citric acid and insoluble calcium sulfate. The resulting solution of crude citric acid is concentrated and filtered to remove calcium sulfate and finally repeatedly crystallized to remove other impurities.
A more modern process uses liquid extraction, activated carbon and multiple recrystallization to recover citric acid.
Electrolysis has been used on a pilot scale to recover citric acid and sodium hydroxide from monosodium acid citrate solution.
Citric acid is used in cosmetics as a buffer to control pH in shampoos, hair rinses and setting lotions. It is also used extensively in food and pharmaceutical products as well as in many industrial applications.
Lactic acid (2-hydroxy propionic acid, C.sub.3 H.sub.6 O.sub.3, molecular weight about 90.08) is made by fermentation of carbohydrates (e.g. sucrose, lactose, cheese whey) by Lactobacillus delbrueckii, L. bulgaricus or L. leichmanii. Primary uses are in foodstuffs and pharmaceutical products in which it has a mild acidic taste in contrast with the sharp taste of some of the other food acids. Lactic acid may be recovered from the above mentioned fermentations by crystallization of calcium lactate followed by conversion of the latter to lactic acid with sulfuric acid. Lactic acid can also be recovered by anion exchange. The latter process suffers from fouling of the exchange resin, production of a waste sodium sulfate stream and the cost of sodium hydroxide and sulfuric acid needed for regeneration of the exchange resin.
Tartaric acid (2,3-dihydroxy succinic acid, C.sub.4 H.sub.6 O.sub.6, molecular weight about 150.09) occurs in the dextrorotary form as potassium hydrogen tartrate ("cream of tartar") in grape juice, wine and wine by-products. Classically the insoluble calcium tartrate has been used to separate tartrate from impurities. Acidification with sulfuric acid is then used to convert calcium tartrate back to tartaric acid, followed by concentration and filtration to remove calcium sulfate. Repeated recrystallization yields pure tartaric acid. The latter is used as an acidulant in carbonated and still beverages including beverage powders as well as in other acidulated food products.
Malic acid (hydroxy succinic acid, C.sub.4 H.sub.6 O.sub.5, molecular weight about 134.09) occurs in apple juice as the levorotary form. It is used in food applications (for example, hard candy) because of its pleasant tartness and flavor-retention characteristics and in non-food applications because of its high solubility in water and its chelating and buffering properties.
Succinic acid (C.sub.4 H.sub.6 O.sub.4, molecular weight about 118.09) occurs with other organic carboxylic acids in most fruit juices. It is used as a flavor enhancer and preservative, as a pH control agent in condiments and relishes, in meat products, in medicinals and in cosmetics.
It will be seen that the five above mentioned organic acids have substantial uses in food products, medicinals and cosmetics and that therefore it is preferable if such acids for such uses are derived from natural sources rather than by synthetic means.
It is therefore an objective of this invention to provide processes and apparatuses for economically recovering free organic acids from aqueous solutions or suspensions in which they occur at least in part as free acid together with other organic matter. It is also an objective to recover such acids from fruit juices (including tomato juices), concentrates thereof or fermentation products thereof without substantially impairing the organoleptic value or the processing and storage characteristics of said juice or derivative thereof.
These objectives and others will become clear from the following description, drawings, examples and claims.
2. Description of the Prior Art
Kilburn et al. (U.S. Pat. No. 3,165,415) describe the recovery of sodium citrate containing excess caustic by Donnan (Exchange) Dialysis or Electrodialysis ("ED") of citrus juices against caustic solutions using anion selective membranes. The repeating units in the apparatus consist of two compartments and alkali is fed to every other compartment. More details of the processes are given by R. N. Smith et al. in "Electrodialysis Processing of Citrus Juice", presented at the Research and Development Associates, Convenience Foods Conference, Philadelphia, Pa., Nov. 18, 64 and by W. K. W. Chen, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, Inc., N.Y., 1965, Vol. 7 page 861. The feed citrus juice typically contained 12 grams soluble solids per 100 grams juice (12 Brix) and 1 gram acidity expressed as anhydrous citric acid per 100 ml of juice. The effluent juice was typically 12 Brix with an acidity of 0.8 grams per 100 ml in the same units. The citrate was recovered in a substantial excess of caustic potash. The high level of pectins and other organic materials present in the juice caused a rapid increase in electrical resistance of the all anion membrane electrodialysis stack. To solve this problem, reversal of the electric current was effected every few minutes without interchanging the juice and caustic potash streams. The superstoichiometric use of expensive caustic potash made this process uneconomical.
As pointed out above, lactic, citric and tartaric acids have been recovered classically in multi-step processes involving precipitation of the calcium salts, resolubilizing with sulfuric acid, multiple recrystallization and purification steps, the latter often with activated carbon and/or ion-exchange. Citric acid has also been recovered on a pilot scale by multiple recrystallizations of its monosodium salt followed by electrolysis of the latter to sodium hydroxide and free acid. Alanine and tartaric acid have been recovered by similar processes. These processes cannot be applied to the recovery of organic acids from fruit juices or fermentation products thereof without seriously interfering with the organoleptic, food and processing values of the juice or fermentation product thereof.